1. Field of the Invention
Exemplary aspects of the present invention relate to an image forming apparatus, a fixing device, and an image forming method, and more particularly, to an image forming apparatus, a fixing device, and an image forming method for fixing a toner image on a recording medium.
2. Description of the Related Art
A related-art image forming apparatus, such as a copying machine, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data by an electrophotographic method. For example, a charger charges a surface of a photoconductor. An optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to image data. A developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor. The toner image is transferred from the photoconductor onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. Thus, the toner image is formed on the sheet.
The fixing device includes a fixing belt, a heating roller, a fixing roller, and a pressing roller. The fixing belt is formed in an endless belt-like shape, and is looped over the heating roller and the fixing roller. The pressing roller opposes the fixing roller via the fixing belt. The heating roller heats the fixing belt. Therefore, the fixing belt may be easily heated even when the fixing roller includes an elastic layer of reduced thermal conductivity.
A rubber roller is generally used as the fixing roller or the pressing roller. The rubber roller includes a core and silicon rubber formed on the core to cover the core. However, the fixing roller or the pressing roller may include silicon sponge of reduced thermal conductivity, as the elastic layer so as to increase a length in a sheet-conveyance direction of a nip formed between the pressing roller and the fixing roller opposing each other via the fixing belt and to prevent or reduce a thermal effect on the fixing belt.
When the silicon sponge is used under high pressure or under conditions in which the silicon sponge is substantially deformed, the silicon sponge may be permanently deformed by compression. To address this problem, in one example of a related-art fixing device, the pressing roller separates from the fixing roller and the fixing belt in a standby mode in which the fixing device does not perform a fixing operation.
Although the silicon sponge can withstand increased pressure temporarily applied while the fixing roller and the fixing belt do not rotate, silicon sponge durability does decrease when the fixing roller and the fixing belt are rotated and thereby deformed repeatedly. Even when the fixing device has a configuration in which the pressing roller separates from the fixing belt or applies reduced pressure to the fixing roller via the fixing belt in the standby mode, a load applied to the fixing roller and the fixing belt might not decrease while the fixing roller and the fixing belt rotate, and thus the durability of the fixing roller might not be improved thereby.
Since a Teflon®-coated surface layer of the fixing belt is soft, a sheet conveyed in the fixing device may generate surface asperities on the surface layer of the fixing belt. The surface asperities may generate gloss stripes on a toner image on a sheet. To address this problem, the pressing roller rotates at a linear speed different from a linear speed at which the fixing belt rotates at the nip formed between the pressing roller and the fixing belt, so as to reduce the surface asperities of the fixing belt. However, when the pressing roller and the fixing belt rotate at the different linear speeds, respectively, a sheet may generate paper dust while the sheet passes through the nip formed between the pressing roller and the fixing belt. The paper dust may scrape the fixing belt, resulting in decreased durability of the fixing belt and the fixing roller including the silicon sponge.